1. Field of the Invention
The present invention relates to a cooling apparatus (hereinafter referred to simply and often as ‘cooler’) adapted for incorporation into a manufacture system and/or an inspection system such as designed to make, inspect and/or evaluate semiconductors, electronic devices or the like, within a space that must be kept severely at a constant temperature.
2. Description of Related Art
Cooling apparatuses are widely used in refrigerators, air conditioners and the like to provide lower temperatures. As well known in the art, each cooler includes a refrigeration circuit built therein such that a compressor, a condenser, an expansion valve and an evaporator are connected to each other in this order by a piping line.
In the refrigeration circuit, successive volumes of a refrigerant in gaseous phase will be compressed continuously by the compressor and transferred to the condenser. The condenser will remove a quantity of heat from each volume of gaseous refrigerant so as to liquefy it into a liquid mass or to produce a mixture of vapor fraction and liquid fraction. Each of successive liquid masses or the fractions will then be delivered to the evaporator through the expansion valve or the like means. The succeeding evaporation process operates to remove a quantity of heat from an ambient load of heat exchange, that is, the object to be cooled, due to latent heat of evaporation of the refrigerant. In other words, each liquid volume of refrigerant will receive heat from the ambient load so as to evaporate again before returning to the compressor.
It is desirable that all the successive volumes of refrigerant returning to the compressor are in a thoroughly vaporized state in order to avoid the so-called problematic ‘compression of a liquid’.
Therefore, the former systems have been designed such that the each of successive volumes of refrigerant, whether being a liquid or a mixture of vapor and liquid, should gasify to a perfect extent.
The expansion valve or the like in the refrigeration circuit of the former system has been controlled to keep the temperature at the outlet of evaporator higher than the temperature of saturated vapor (herein called ‘saturation vapor temperature’) of the refrigerant. In more detail, degree of superheating of the refrigerant vapor has been regulated to be constant at the evaporator outlet, by control of an expansion valve or the like.
Disclosed in Japanese Patent Laying-Open Gazette No. 61-89456 (called Gazette '456 hereinafter) is a proposal that was made to return every volume of refrigerant to the compressor in its completely evaporated state.
The countermeasure described in the Gazette '456 employs heat exchange between a lower-pressure side and a higher-pressure side, with the former side flowing between the evaporator and the compressor, and the latter one between the compressor and the expansion valve. The refrigerant fraction having left the evaporator will thus be superheated before entering the compressor.
Such a proposal of the Gazette '456 is directed to a perfect vaporization of refrigerant within the evaporator. The heat exchange between the said lower and higher pressure-sides are provided so as to superheat the refrigerant vapor discharged from the evaporator so that even a very small amount of liquid fraction will not come back into the compressor at all.
Also in the former coolers, temperature control of their portions in contact with the ambient load of heat exchange has been effected by switching on or off the compressor.
If the temperature of ambient load-contacting portions is above a target temperature, then the compressor will be actuated in the cooler to lower the temperature of evaporator so as to more cool said portions. If contrarily the temperature of said ambient load-contacting portions is below the target temperature, then the compressor will be switched off.
The on-off control of the compressor for keeping the temperature of ambient load-contacting portions close to the target temperature has however given rise to a certain problem. Such a simple mode of control has often caused fluctuation or ‘hunting’ of said portions' temperature.
Some plants for manufacturing, inspecting and/or evaluating semiconductors, electronic components and the like, or some kinds of testers for environmental factors, must be ‘thermoregulated’ strictly. However, such an on-off control of compressors as summarized above will fail to realize a sufficient stability in temperature of the coolers' portions in contact with ambient load, with a resultant significant fluctuation in temperature of the object to be controlled, making it difficult to meet the severe requirement.
Some of those plants are therefore equipped with highly responsible heaters in addition to the coolers so that an excessive degree of cooling is canceled with heat which those heaters will emit.
This approach to diminish the problem inherent in the former systems does however include a contradiction that cooling is done on one hand and heating is done simultaneously on the other hand, thus lowering efficiency of energy and failing to save energy.
One of the most important requirements to the environmental testers and the like is that temperature distribution in each of them should be as small as possible, but the former apparatuses have not been satisfactory from this point of view.
For example, the environmental testers and the like have to operate within a very wide range of target temperatures that may be changed from −40° C. to +100° C. A cooler installed in such testers or the like has had to include an evaporator of such a high duty (viz., high efficiency of heat exchange) as matching the lowermost target temperature of system. If temperature of ambient load (for example in a temperature chamber) is considerably high, then the refrigerant contained in the evaporator will gasify soon and immediately, absorbing its whole latent heat amid the evaporator. As a result, temperature will become non-uniform even by means of the evaporator itself included in this type of coolers, thereby producing an unallowable temperature distribution throughout the ambient load, which disables accurate environmental tests.
In order to diminish temperature distribution in the ambient load of heat exchange, a supplementary or auxiliary coolant (such as brine) may be involved and controlled in temperature by the cooler before supplied to the ambient load. However, such a countermeasure necessitating its own circuit, circulating pumps and other devices for the supplementary coolant will render the cooler expensive and make the cooler too large in scale to be accommodated in a given space not so wide.
FIG. 11 illustrates an example of this system employing therein the supplementary coolant. Its main portion is a cooler that is composed of a compressor 100, a condenser 101, an expansion valve 102 and an evaporator 103. In addition to these principal devices, a reservoir 105 is indispensable in such a proposal and will further need an electric heater 106 immersed therein, thus raising an equipment cost, occupying a larger space and unreasonably lowering thermal efficiency.